The signal from a GPS (Global Positioning System) sensor or other satellite-based navigation system located in a device carried by a person on the wrist or elsewhere on the body, has a very small bias error, i.e. a systematic error, but contains a great deal of noise. At a measurement frequency of 1 Hz with the person walking or running, the noise in a purely GPS-based speed measurement can be in the order of 20-30%, compared to the pure GPS signal.
Direction or speed data measured and estimated from sensors carried directly on the body of a person, contains usually less noise. However, large bias errors may be present in these measurement signals.
Sensor fusion means that first and second sensors may be based on different operating principles, but they are measuring the same physical variable. For example, horizontal speed can be measured using a satellite-positioning sensor or an acceleration sensor. Sensors that can be used in sensor fusion may be, for example, a GPS sensor, a magnetometer (compass), and acceleration sensors. With such sensors, the acceleration, speed and direction of a moving object may be measured and displayed on a mobile device, for example on the display of a wristop computer, a mobile telephone or any other portable device.
It is known from GB 2497153 to measure first and second physical variables with first and second sensors respectively, and to determine an estimate of a target variable by measuring a first physical variable. An error estimate is determined by measuring a second physical variable, and the estimate of the target variable is filtered with a strength that depends on the error estimate. However, during a loss of satellite signals, as may be the case in shadow areas such as in tunnels, backyards and mountain areas, where only a weak or non-detectable positioning signal strength exists, a satellite-based measurement may not be available at all.
Therefore, there is a need for a method and system capable of determining the direction of movement of an object. The method and system should be, for example, usable for a positioning system that delivers accurate and uninterrupted position data and other data derivable therefrom. In view of the foregoing, it would be beneficial to provide a method and a system capable of determining the direction of movement of an object comprising a first part and a second part which is cyclically moving relative to the first part, in which method and system the battery consumption can be reduced. It would be also beneficial to provide a method and system capable of providing uninterrupted position data and other data derivable therefrom.